1. Field of the Invention
The present invention relates to a wavelength selective optical switch which separates a wavelength division multiplexed light for each wavelength by a spectral element, and thereafter, condenses the separated lights using lenses to reflect the condensed lights by movable mirrors, to thereby switch optical paths for the lights of respective wavelengths, and to an optical device provided with a spectroscopic function utilizing the lights of respective wavelengths condensed on the lenses.
2. Description of the Related Art
At present, the optical network with the wavelength division multiplexing (WDM) communication as the core thereof is being progressed at a high pace, in order to accommodate drastically increasing Internet traffics. The current WDM communication is in the network mode of point-to-point type. However, it is considered that, in the near future, the network mode of the WDM communication will be developed to the ring type network and the mesh-shaped network, and at each node configuring these networks, the processing, such as, the adding/dropping of arbitrary wavelengths, the all optical cross connecting (OXC) without the conversion into electricity and the like, will become possible, so that the dynamic setting/canceling of optical paths based on wavelength information will be performed (refer to the literature: “IP Over-photonic Network Vision (2) High Technology of Photonic Backbone Network” by Kenichi Satoh, et al., Journal of The Institute Of Electronics, Information And Communication Engineers, February 2002, pp. 94-103).
There has been proposed a wavelength selective optical switch as shown in FIG. 11 for example, as an optical switch capable to be arranged on each node of the photonic network which maximizes the optical technology as described in the above (refer to U.S. Pat. No. 6,549,699). This conventional wavelength selective optical switch comprises: an input and output optical system 110 consisting of an input port Pin and output ports Pout1 to Pout3; a spectral element 120; a condenser lens 130; a mirror array 140 in which a plurality of movable mirrors is arranged; and a base 150 on which the above described optical parts are mounted. In the wavelength selective optical switch having the above configuration, a WDM light input to the input port Pin is separated into lights of respective wavelengths by the spectral element 120. Thereafter, the separated lights are condensed by the condenser lens 130 on the movable mirrors respectively corresponding to the respective wavelengths, in the mirror array 140, and reflecting surface angles of the movable mirrors are controlled, so that reflected lights of respective wavelength are guided to arbitrary output ports Pout1 to Pout3, and the switching of optical paths for the respective wavelengths is performed.
As the spectral element 120 used in the conventional wavelength selective optical switch as described above, typically, a diffraction grating is utilized. The diffraction grating is an optical element made up by forming multiple parallel grooves uniformly on a glass substrate, and is capable of emitting lights of plural wavelengths which are incident at a fixed angle, at different angles for each wavelength utilizing an optical diffraction phenomenon. Therefore, the separation of wavelengths can be performed.
Further, as the mirror array 140, the one configured by arraying mirrors formed by the MEMS (Micro Electro Mechanical Systems) technology (to be referred to as MEMS mirrors hereafter) is typically utilized, and one MEMS mirror is arranged for the light of one wavelength separated by the spectral element 120. The MEMS mirror has a structure in which an inclination angle of the reflecting surface thereof is variable due to the electromagnetic force, and therefore, as shown in FIG. 12, the output port to which the reflected light is guided is decided according to the inclination angle of the reflecting surface.
One of the indexes showing the performance of the wavelength selective optical switch as described above is the transmission band. As shown in FIG. 13, this transmission band becomes broader, as a ratio (W/ω) between a beam diameter ω of the light condensed on the MEMS mirror corresponding to each wavelength and the mirror width W is larger and also the deviation of center wavelength is smaller. Namely, as the width W of the MEMS mirror is broader, the beam diameter ω on the MEMS mirror is smaller and also a condensing position of the light corresponding to each wavelength of the ITU grid is more coincident with the center of the MEMS mirror, the transmission band becomes broader. Here, the ITU grid is the wavelength standardized by the International Telecommunication Union. If the transmission band of the wavelength selective optical switch is broad, there are advantages in that an upper limit of bit rate capable to be coped with is increased and the number of multistage connections of the wavelength selective optical switches can be increased. In other words, if the transmission band of the wavelength selective optical switch is narrow, excellent transmission characteristics cannot be ensured.
In order to achieve the sufficient transmission band characteristics by the wavelength selective optical switch as described above, it is necessary to make the condensing positions of the lights corresponding to the respective wavelengths of the ITU grid coincident with the centers of the MEMS mirrors respectively corresponding to the wavelengths.
To be specific, as shown in FIG. 14 for example, provided that a certain wavelength of the ITU grid is made to be a reference wavelength λ0 and a wavelength being ±i-th (i=1, 2, . . . ) to the reference wavelength λ0 is represented by λ±i, when the WDM light containing the lights of the wavelengths λ0 and λ±i is given at an incident angle α to the spectral element 120 which uses the diffraction grating, an angle θi between the light of the wavelength λ0 and the light of the wavelength λ+i emitted from the diffraction grating is expressed in accordance with the following formula (1).θi=Arc Sin(N×λi−Sin α)−Arc Sin(N×λ0−Sin α)  (1)In the above formula, N is the number of grooves per 1 mm of the diffraction grating, and the order of diffraction is the first order.
Based on the relationship in the formula (1), an interval Xi between the lights of the wavelengths λ0 and λi condensed on the mirror array 140 (to be referred to as a beam pitch hereafter) can be obtained by the following formula (2), provided that a focal distance of the condenser lens 130 is f.Xi=f×θi=f×{Arc Sin(N×λi−Sin α)−Arc Sin(N×λ0−Sin α)}  (2)
In the above formula (2), since λ0 and λi are always fixed, the beam pitch Xi of the lights condensed on the mirror array 140 serves as a function for the focal distance f of the condenser lens, the number of grooves N of the diffraction grating and the incident angle α of the WDM light. Accordingly, a distance X0i between the MEMS mirror corresponding to the wavelength λ0 and the MEMS mirror corresponding to the wavelength λi (to be referred to as a mirror pitch hereafter) is decided to be coincident with the beam pitch for the case where the focal distance f, the number of grooves N and the incident angle α become ideal values f0, N0 and α0. Namely, the mirror pitch X0i is previously designed in accordance with the following formula (3).X0i=f0×{Arc Sin(N0×λi−Sin α0)−Arc Sin(N0×λ0−Sin α0)}  (3)
However, in the conventional wavelength selective optical switch as described in the above, if an error occurs in the focal distance f of the condenser lens, in the number of grooves N of the diffraction grating, or in the incident angle α of the WDM light, since the deviation occurs between the beam pitch of the lights of respective wavelengths actually condensed on the mirror array 140 and the mirror pitch of the MEMS mirrors in the mirror array 140, there is caused a problem of the degradation of the transmission band. Since each MEMS mirror arranged on the mirror array 140 is formed by performing the fine processing, such as etching or the like, on a silicon substrate, it is hard that the mirror array 140 in itself has a mechanism for adjusting the mirror pitch.
Namely, in order to achieve the sufficient transmission band characteristics in the conventional wavelength selective optical switch, the beam pitch of the lights condensed on the mirror array 140 needs to be always coincident with the mirror pitch in the mirror array 140. As a method of realizing such necessity, there is considered, for example, a method of tightening the tolerance of the focal distance f, the number of grooves N and the incident angle α up to a degree at which the degradation of the transmission band is allowable. Further, there is considered a method of correcting the error as described above by disposing a rotation mechanism to the diffraction grating to make the incident angle α variable, even if the error occurs in the focal distance f or in the number of grooves N.
However, in both of the above described methods of making the beam pitch Xi coincident with the mirror pitch X0i, there still remains a common problem in that the precision required to the incident angle α is significantly strict. For example, in the case where the focal distance f of the condenser lens is 100 mm, the number of grooves N of the diffraction grating is 1800 /mm, the incident angle α is 66.50°, the usable wavelength range of the WDM light is C-band (1530 to 1565 nm), the wavelength spacing is 100 GHz and the number of wavelengths is 44 waves, if the degradation of the transmission band is to be suppressed to 3 GHz or less, the precision required to the incident angle α is 0.01° or so. Such precision of the incident angle α is hard to be realized even if the diffraction grating is fixedly mounted as in the former method, or even if the rotation mechanism is disposed to the diffraction grating as in the latter method. Even if the precision of the incident angle α could be realized at 0.05° by adjusting the diffraction grating by the rotation mechanism, the degradation of the transmission band at that time is 15 GHz or more, and such a value is problematic in the operation of the wavelength selective optical switch.
In fact, in the conventional wavelength selective optical switch, there has been a problem in that the degradation of the transmission band is significant in the typically realizable mounting precision and adjusting precision, and therefore, the sufficient characteristics as the wavelength selective optical switch cannot be achieved.